Commonly used optical fiber is: silica-based; of outside dimension ˜125 microns (μm); with “up-doped” core; and often undoped cladding. Initially introduced multimode fiber, with its core size of 25-50 μm, now occupies but a niche position—that of short and intermediate distance communications with operation at the nominal system wavelength of 850 μm. At this time, longer-distance communications is based on single-mode fiber—structure of the same outside diameter, but now of reduced core size. Single-mode fiber, with its 1-6 μm radius core, is designed to support solely the first or “fundamental” mode. Higher-order modes, not present in the initially-launched laser pulse, but yielded, e.g., by encounter with unintended scattering centers, are not supported in such core structures, and, accordingly, are ultimately radiated from the fiber. (Content of such spurious higher-order modes—continuously generated by encounter with fresh scattering centers, and continuously removed by radiation—in usual single-mode structures, attains some small steady-state population that may be ignored for most purposes.)
Single-mode fiber and systems, in retaining dominance, have undergone many iterations. Usual single-mode systems are based on laser transmitters operating at wavelengths in either of two low-loss wavelength regions associated with the silica-based glass of generally-used optical fiber. Operation had, for some years, been at the nominal system wavelength of 1310 nanometers (nm), defining one such region. Nature looked kindly on 1310 nm operation, in offering fiber of generally low dispersion, as desired for small pulse-spreading and permitted high bit-rates. (Following accepted design principles for 1310 nm fiber, the two opposite-sign contributions to total chromatic dispersion—the “waveguide dispersion” associated with the design of the fiber, and the “material dispersion” associated with the bulk characteristic of the glass composition itself—substantially compensate.)
Operation within the lower-loss 1550 nm region, however was complicated by relatively high levels of chromatic dispersion and consequent limitation on bit-rate. That problem was solved by introduction of Dispersion Shifted Fiber (DSF), which, with its larger waveguide dispersion, enabled compensation of the larger material dispersion at that wavelength. DSF was, in turn, displaced by Non Zero Dispersion Fiber (NZDF), e.g., TrueWave® fiber, with its precisely-determined small but finite value of chromatic dispersion, balancing the needs of Wavelength Division Multiplexing (WDM) (dispersion both sufficiently large for periodic phase cancellation to control Four Wave Mixing (4WM) responsible for channel-to-channel cross-talk, and sufficiently small to limit pulse spreading and enable high per-channel bit-rate). (With total absence of chromatic dispersion, DSF assured identical phase velocity for all WDM channels, thereby avoiding phase-cancellation, allowing unlimited buildup of spurious signal, and precluding the increased capacities expected from multi-channel operation.) Use of NZDF has permitted demonstration of a 40-channel, single mode fiber system, of trillion bit/sec capacity.
Existing single-mode systems fall into three categories: 1. Enterprise Networks (Campus Local Area Networks or “LANs”)—of span lengths 1-3 kilometers (km), and Subscriber Distribution (connecting the central office to the subscriber)—of span lengths 1-20 km; 2. Metropolitan Networks (linking nearby central offices)—of span lengths 5-40 km; and 3. Long-distance networks—typically of span length up to 100 km before signal amplification or regeneration.
Likely improvements in future single-mode systems will address economic issues, as well as performance. Small mode field radius (MFR), implicit in traditional single-mode fiber, leads to high power density in the fiber core, thereby increasing consequence of non-linearities, restricting introduced power and, accordingly, limiting distance between optical amplifiers. The same consideration impacts the number of channels in a WDM channel set. Small core size imposes a high degree of needed precision, both in fiber fabrication and in system installation—all with cost implications. Substantial macrobending susceptibility imposes space constraints, requiring extensive storage space. Significant microbending susceptibility, with resulting cabling loss, limits choice of fiber coatings—requires now-prevalent dual coatings (with soft inner coating to buffer the fiber from inner surface roughness, and harder outer coating for abrasion-resistance). The same dual coatings increase space requirements in cable design and in required duct space.
Fabrication advances have been impressive. Intractability of the high-melting, and, consequently easily-contaminated, silica-based fiber, has yielded to suitable manufacturing processes, which maintain product within extremely tight compositional, dimensional, and purity specifications. Low-loss dopants/doping processes, for tailoring index-of-refraction and imparting wanted light-guiding properties, have been developed.
Common manufacturing processes are: Modified Chemical Vapor Deposition (MCVD); Outside Vapor Deposition (OVD); and Vapor Axial Deposition (VAD). Described, e.g., in Optical Fiber Telecommunications, S. E. Miller and A. G. Chynoweth, 1979, Academic Press, Chapter 8, all react gaseous silicon halide-containing material with oxygen to produce an initial particulate, silica-containing “soot” body, of carefully-controlled composition, which is, thereafter consolidated to yield the body, constituting at least the critical core precursor, from which the fiber is ultimately drawn. MCVD and OVD achieve critical core profiling by means of layer-by-layer, longitudinal deposition of thin, uniform-composition layers of material—with layer-by-layer composition changed or unchanged, containing index-increasing or index-decreasing dopant, as needed. Preform preparation may entail further processing such as etch-removal of temporary substrate—of the outer MCVD deposition tube or the OVD mandrel. Resulting hollow MCVD and OVD bodies are collapsed to make the solid preform. MCVD manufacture often incorporates a cost-reducing procedure, by which the deposited body is placed within an outer cladding tube, of less critical, relatively inexpensive material, to produce the (composite) preform. VAD depends on “end-on” growth of compositionally-graded material.
Co-pending U.S. Patent Applications
Co-pending patent application “Optical Fiber For Single-Mode Operation,” Ser. No. 10/407,376, filed on even date herewith and assigned to the assignee of this application, and which is hereby incorporated by reference, describes and claims a novel single-mode fiber structure, “Enhanced Single-Mode Fiber” (ESMF). In accordance with that application, mode-stripping of higher-order modes enables single-mode operation in fiber cores of sufficient size for limited multimode operation (for “few-mode” operation)—in fiber cores of sufficient size to support a limited number—generally a total of from two to four modes at a system wavelength. Fiber of that invention benefits from larger core size relative to traditional design, as well as from greater wavelength transmission capability (now for operation over a spectrum including wavelengths below cutoff in traditional design). The mode-stripping phenomenon, common to all species, is an outgrowth of a proposed remedy for bandwidth loss in early multimode fiber—i.e., the “mode-mixing” phenomenon, dependent on successive perturbations in refractive index along the fiber, as “seen” by a travelling pulse. Such fiber perturbations were proposed for encouraging mode conversion among the supported core modes, thereby “averaging” traversal times for the various modes, limiting pulse spreading, and improving bandwidth. Commercial use of such perturbed fiber has been limited due to added loss in signal strength accompanying the intended mode-mixing.
Another co-pending patent application, “Enhanced Multimode Fiber,” Ser. No. 10/408,476, filed on even date herewith and assigned to the assignee of this application, addresses that added loss, and, thereby, reinstates mode-mixing as a viable mechanism for alleviating the bandwidth reduction due to mode dispersion. Mode dispersion, recognized as originating with generation of spurious modes by scattering centers, was to have been alleviated by vastly increasing incidence of mode conversion (of “mode coupling” between modes), so that modes making up an individual pulse of light would have spent an equal amount of time as every other mode, thereby averaging modal traversal time. This co-pending application first identifies the added signal loss as due to unwanted coupling with “cladding modes” (with modes of such high order as not to be supported in the fiber core—therefore, ultimately to be lost by radiation from the fiber). The claimed Enhanced Multimode Fiber (EMF) avoids this added loss by decoupling a final core mode, thereby arresting step-wise coupling of modal energies with cladding modes.
ESMF depends on a corollary of the EMF thesis—rather than preventing coupling with cladding modes, it provokes coupling of all but the fundamental mode with cladding modes, thereby inducing loss of higher-order modes and approaching the single-mode operation of conventional single-mode fiber.